Tuna is always wild-caught, but the canned in the stores is often screwed up with water-packing or packing in soy oil or some such. In August and Sept. look for Copper River Salmon -- really delicious wild-caught. There may still be frozen available later in the year. And don't forget shell fish -- crab is great. Shrimp, you need to watch the source (check the MDA archives for this). Also check out bi-valves like clams and oysters.

Yes, farm rasied fish is often fed corn and other grains and then dyed to enhance color (ie. make the salmon more pink since they lose the stuff in their natural diet that would normally make them pink). Farmed fish also live in small confinement with many other fish and lots of fish waste. Farmed fish = CAFO beef, IMO.

While I agree that wild-caught fish is likely to be superior, this primarily applies to the fattier fish - just like the n3:n6 ratio is vastly different in grass fed versus conventional beef, but there isn't really enough fat in a skinless/boneless chicken breast to actually worry about, the same is true with fish - there's a big difference between farmed and wild-caught salmon(farmed still has quite a bit of n-3, but a lot more n-6) and other relatively fatty fish, but for very low-fat fish the distinction isn't quite as important. Basically, if you're buying fish for the omega-3's, absolutely buy wild. For other fish, you have an option, but I'd still pay attention to the origin.

If you do buy farmed, however, I'd check the origin. I've seen farmed fish from China(mostly canned fish) - and while I don't have an objection to Chinese goods in general, I would NEVER, EVER touch fish farmed in China. Mostly because the one of the areas of China that farms the most fish to my knowledge(Guangdong province) also happens to be one of the dominant areas for export manufacturing, including the electronics sector - and these fish farms are often not all that far from the factories. Knowing some of the chemicals involved in those industries, I'm not about to trust fish farmed in China under any circumstances.

In general, wild-caught Alaskan salmon is my first choice for both taste and health reasons; if it's not in-season I tend to expand my selection a bit.